A method and device for accurately easily and accurately positioning patients with breast cancer in the prone position for imaging and radiotherapy is disclosed. The device is a couch or support structure with a double-layer structure that allows relative planar motion between the two layers. To easily slide the top layer, an array of ball transfer units is embedded on the upper side of the bottom layer while the under side of the top layer has a corresponding array of bearing plates to prevent the ball transfer units from grinding into the top layer. A locking mechanism is specially designed to interlock the two layers safely when a knob handle is tightened. A frame of bars is attached to the bottom layer to constrain translation of the top layer. Each layer has two openings for the through insertion of breasts. When a patient lies in the prone position, the breast to be treated is pendent through the couch, and the untreated breast may be blocked from radiation exposure by a removable opening cover.
